The first tell-tale signs appeared under the collar of the prime minister's pale blue shirt, seeping down his chest under his tie and then creeping across his midriff.

When Tony Blair's jacket opened as he raised his arms at the end of the 56 minute speech, his shirt looked as if someone had thrown a glass of water at it.

As wags suggested that his speech was more about perspiration than inspiration, Mr Blair gave his wife, Cherie, a damp hug. Last night the talk of Brighton was the issue of body temperature rather than the content of the leader's speech.

Downing Street and Labour party spin doctors went into action, aware that it was damaging for their man to look as if he was breaking out in a sweat under pressure.

"It proves he is a real man," said one spinner, while another added: "It showed that when the heat is on he gets going."

The public relations consultant Max Clifford said: "I have spoken to people who have been to the conference and they have all told me how incredibly hot the conference centre is.

"So it wasn't as if Mr Blair's advisers could not have anticipated this.

"In my opinion, they made a critical mistake.

"This will now give the Tories a green light to say Mr Blair was sweating because he was squirming and fighting for his political life.

"They certainly won't be mentioning the heat."

The prime minister is believed to have showered and changed his shirt in Brighton last night before going to a Labour fundraising dinner.